Datasheet
TPS65708
www.ti.com
SLVSAE1A –OCTOBER 2010– REVISED FEBRUARY 2011
180° OUT-OF-PHASE OPERATION
In PWM Mode, the converters operate with a 180° turn-on phase shift of the PMOS (high side) transistors. This
prevents the high-side switches of both converters from being turned on simultaneously, and therefore smooths
the input current. This feature reduces the surge current drawn from the supply.
UNDERVOLTAGE LOCKOUT / ENABLE for DCDC1, DCDC2, LDO1, and LDO2
The undervoltage lockout circuit prevents the device from malfunctioning at low input voltages and from
excessive discharge of the battery. It disables the dcdc converters and LDOs at too low input voltages.
As TPS65708 does not have enable pins for the dcdc converters and LDOs, the internal undervoltage lockout
not only serves as a protection circuit but also as an enable circuitry. The supply voltage to TPS65708 is
internally sensed at pin V
CC
. When the voltage at V
CC
exceeds 3.6V, the internal enable signals to the dcdc
converter and LDOs are set HIGH to start-up the outputs in the pre-defined sequence. When the supply voltage
drops below 3.6V, the dcdc converters and LDOs are disabled again and the discharge circuitry is enabled to
make sure the voltage at the output capacitor ramps down quickly. Disabling the dcdc converter or LDO, forces
the device into shutdown, with a shutdown quiescent current as defined in the electrical characteristics. In this
mode, the power FETs are turned-off and the entire internal control circuitry is switched-off.
OUTPUT VOLTAGE DISCHARGE
The dcdc converters and LDOs contain an output capacitor discharge feature which makes sure that the
capacitor is discharged when the supply voltage drops below the undervoltage lockout threshold. The discharge
has a built in delay function, so the output discharge is active for a couple of 100ms after the V
CC
voltage
dropped below its undervoltage lockout threshold. This will make sure that the cap is discharged even the supply
voltage dropped below 2.1V. The discharge function is also enabled when voltage is applied at V
CC
starting at
about 2.1V until the voltage exceeded the undervoltage lockout threshold that enables the power-up sequencing.
POWER-UP SEQUENCING
There are 3 different power-up sequencing options available. The options are factory set and can not be
changed by the user. Contact TI if an option different from the default is needed.
1. DCDC1, DCDC2, LDO1, and LDO2 are turning on at the same time.
2. DCDC1 first, when power good, LDO1 is enabled, when power good, DCDC2 is enabled when power good,
LDO2 is enabled
3. DCDC2 first, when power good, LDO2 is enabled, when power good, DCDC1 is enabled when power good,
LDO1 is enabled
TPS65708 is set to option 2 such that DCDC1 starts first followed by LDO1, DCDC2, and LDO2. See also the
timing diagram under the table of graphs.
SHORT-CIRCUIT PROTECTION
All outputs are short circuit protected with a maximum output current as defined in the electrical specifications.
THERMAL SHUTDOWN
As soon as the junction temperature, T
J
, exceeds typically 150°C for the dcdc converters or LDOs, the device
goes into thermal shutdown. In this case, the low side and high side MOSFETs for the dcdc converters as well
as the LDOs are turned-off. The device continues its operation and powers up the dcdc converters and LDOs
with the pre-defined sequencing when the junction temperature falls below the thermal shutdown hysteresis
again. During thermal shutdown also the LED driver is disabled.
LDOs
The low dropout voltage regulators are designed to operate well with low value ceramic input and output
capacitors. They operate with input voltages down to 1.7V. Both LDOs offer a maximum dropout voltage of
300mV at rated output current. The LDOs support a current limit feature.
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